Lit., the tombs, the unseen ones. The word ἄδηλος, unapparent, occurs only here and 1Co 14:8, of the trumpet giving an uncertain sound.
That walk over (περιπατοῦντες)
The participle, and without the article; and therefore better, as they walk; walk about (περί) on their daily business. In Matthew the sepulchres are whitened, that men may see them and avoid ceremonial defilement. Here they are not seen, and men walking on them are unconsciously defiled. See on Mat 23:27.